This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Cargo is generally transferred from one port location to another port location by vessels, such as carriers. These carriers have propulsion and navigation systems for movement across large bodies of water, which may be referred to as open seas. In addition, the carriers may include accommodations for marine operations and storage tanks for liquid cargo. For example, with some carriers, special equipment and systems may be installed to assist with the transport of specific cargo, such as LNG. As such, the systems on carriers provide a mechanism for economically transferring cargo between market locations.
As an example, after natural gas is produced from a well, it is processed and may be liquefied at export terminals or other facilities to convert it into liquefied natural gas (LNG). LNG is the basis of a delivery technology that allows remote natural gas resources to be economically delivered to other markets. The LNG is shipped to market in specially-designed LNG carriers (LNGC) that are configured to store and transport the LNG across the large bodies of water. Then, the LNG is converted back from LNG to natural gas at an import terminal near the market locations. Typically, the import terminals are located onshore at a port location or offshore near a port location. Regardless, the import terminal is connected through a pipeline to onshore equipment for further processing or distribution.
Offshore terminals may be beneficial because the terminals do not utilize onshore property and in an offshore environment, security concerns may be reduced. One concept for an offshore terminal is a floating storage and regasification unit (FSRU). FSRU is a dedicated, moored offshore structure that transfers cryogenic LNG with LNGCs, stores the LNG in storage tanks, regasifies the LNG using heat exchangers, and delivers the natural gas to a pipeline coupled to the import terminal. The FSRU concept generally includes cryogenic cargo transfer equipment and LNG vaporization facilities, which may be located on the platform of the FSRU.
However, offshore environmental conditions may be a factor that limit the time periods that the LNGCs and FSRUs can operate. For instance, harsh environmental conditions may provide periods of time where connecting the LNGCs and FSRUs cannot be done safely and reliably. Further, if the offshore environmental conditions are too severe to allow the LNGCs and FSRUs to remain connected, then the FSRUs can only deliver natural gas to the pipeline from its stored reserves. Further, if the stored reserves on the FSRUs are depleted, then natural gas delivery is stopped to the pipeline. Intermittent service or interruptions to the flow of natural gas into or from a pipeline may result in penalties and cost increases to terminal operators.
To address the environmental conditions, various offloading approaches are utilized to transfer LNG between LNGCs and FSRUs. For instance, one offloading approach is side-by-side offloading which is currently employed at land-based import and export terminals. Side-by-side offloading is performed with the LNGC and FSRU arranged in a side-by-side configuration with the LNG transfer occurring between conventional mechanical loading arms located near amidships of the FSRU and an offloading manifold on the LNGC. Because of the limitations on the movement of these loading arms and the relative motions between the LNGC and the FSRU, conventional land-based cargo transfer using mechanical loading arms is typically performed in protected waters with the significant wave height less than or equal to 1.5 meters.
A second offloading approach is tandem offloading. Tandem offloading is based on existing technology used to transfer oil between floating production storage and offloading (FPSO) vessels and shuttle tankers. In tandem offloading, the two vessels are arranged bow-to-stern, and the LNG transfer is achieved using flexible hoses or mechanical devices like pantographs. For LNGCs, the flexible cryogenic hoses or large loading arms, which are called booms, are utilized to transfer the cryogenic LNG with the LNGC carrier's bow located behind the stern of the FSRU. With the flexible cryogenic hoses, the tandem offloading approach may remain operable in more severe seastates, such as 2.5 to 3 meter significant waves, than the side-by-side offloading approach.
A third offloading approach is subsea LNG transfer system (SLTS) offloading approach, which is referenced in International Patent Application No. WO2006/044053. In the SLTS offloading approach, the LNGCs and FSRU are connected over a distance of about 2 kilometers (km) by subsea cryogenic risers and pipelines. The LNGC is connected to a floating cryogenic buoy and transfers the LNG through the buoy and one or more flexible cryogenic risers and pipelines to another buoy located at the FSRU. Because the LNGCs and FSRU are separated and may move independently, the SLTS may operate for more severe seastates, such as 4 to 5 meter significant waves. Accordingly, each of these offloading approaches may be utilized to maintain uniform delivery of NG to the pipeline, which is often part of gas marketing contracts.
However, the use of FSRUs with any of these offloading approaches suffers from technical and commercial limitations. For instance, because the FSRUs are permanently moored with no access to dry dock maintenance, a large infrastructure and associated capital expenditure is typically involved with any permanently-moored FSRU. This large initial capital expenditure results in a significant reduction in the overall LNG delivery chain economics. Also, additional equipment and operations, such as dedicated positioning tugs or navigation systems on the LNGCs, are involved to facilitate berthing operations for the LNGCs with the FSRU. While improved relative to onshore terminals, FSRUs still pose a security threat and have to be managed to address the open access provided in an offshore setting. Further, for certain offloading approaches, such as the SLTS approach, each of the LNGCs have to be modified with a turret to accommodate the buoy leading to increased costs for the entire LNGC fleet.
An alternative to the FSRU-based import or export terminal is to include the regasification equipment on the LNGC. See U.S. Pat. No. 6,089,022. These vessels are LNGCs with extensive modifications to allow shipboard regasification of the LNG and offloading of the natural gas through a conventional natural gas offloading buoy into the pipeline. These carriers, which may be referred to as regasification LNGCs, are equipped with traditional LNGC offloading equipment (e.g. a manifold to accept loading arms) to interact with conventional LNGCs. Disadvantageously, the capital expenses of these regasification LNGCs may be significantly larger than traditional LNGCs because each regasification LNGC is modified with heat exchangers for regasification operations, a turret for offloading to the gas buoy, and reinforced cargo tanks to withstand sloshing loads. In addition, the storage of the regasification LNGCs is limited because the regasification facilities are configured within a vessel designed for efficient transit over long distances.
As such, a method or mechanism for enhancing delivery of cargo, such as NG and LNG, in an efficient manner is needed. In addition, this method or mechanism may avoid the problems associated with onshore terminals, offshore FSRUs, and/or the use of regasification LNCGs over long distances.
Other related material may be found in at least U.S. Pat. Nos. 3,590,407; 5,501,625; 5,549,164; 6,003,603; 6,089,022; 6,637,479; 6,923,225; 7,080,673; 6,546,739; U.S. Patent Application Publication No. 2004/0187385; U.S. Patent Application Publication No. 2006/0010911; European Patent Application No. 1,383,676; International Patent Application No. WO 01/03793; International Patent Application No. WO2006/044053; Loez, Bernard “New Technical and Economic Aspects of LNG Terminals,” Petrole Information, pp. 85-86, August 1987; Hans Y. S. Han et al., “Design Development of FSRU from LNG Carrier and FPSO Construction Experiences,” Offshore Technology Conference May 6-9, 2002, OTC-14098; “The Application of the FSRU for LNG Imports,” Annual GAP Europe Chapter Meeting September 25-26, 2003; O. B. Larsen et al., “The LNG (Liquefied Natural Gas) Shuttle and Regas Vessel System,” Offshore Technology Conference May 3-6, 2004, OTC-16580; and Excelerate Energy (visited on Oct. 24, 2006) <http://www.excelerateenergy.com/activities.php>.